The present invention relates to a cable transmission system, or more in particular to a variable equalization system suitable for a .sqroot.f equalizer requiring a wide range of variable area.
Conventional .sqroot.f equalization systems operate on the principle described below. As explained in "High-Speed PCM" authored and compiled by Yoshiharu Shigei, 1975, p. 218 to p. 219, when the cable loss undergoes a change from the standard loss of a .sqroot.f (dB) by .DELTA.a.sqroot.f (dB), the resulting loss is expressed by equation (1). ##EQU1## This is equivalent to the frequency characteristic having moved parallelly along the frequency axis a number ##EQU2## of times. In spite of the variable equalization thus effected by parallel relocation of the frequency characteristic, the variable range of gain, which is limited by the amount of variation of a variable element used in a variable amplifier, is narrow.
In the prior art described above, the general .sqroot.f characteristic is changed by equivalently displacing the position of a natural frequency of an amplifier with a fixed gain. As a result, the variable characteristic range is limited by the amount of change of a variable element.
When a variable-capacity diode is used as a variable element, for example, the variable range is limited by the ratio between maximum and minimum capacities. In order to overcome this difficulty, a fixed attenuator is inserted with a false line to enlarge the variable range in the prior art systems. This makes it necessary to insert or remove the false line in accordance with an applicable line, thereby causing a difficulty in the maintenance, line design, etc.